An artery is one of the tube-shaped blood vessels that carry blood away from a heart to the body's tissues and organs. An artery is made up of an outer fibrous layer, a smooth muscle layer, connecting tissues and inner lining cells. If arterial walls become hardened due to the accumulation of fatty substances, then blood flow can be diminished. Hardening of the arteries, or loss of vessel elasticity, are termed arteriosclerosis while fatty deposit build-up is termed atherosclerosis. Atherosclerosis and its complications are a major cause of death in the United States. Heart and brain diseases are often the direct result of this accumulation of fatty substances that impair the arteries' ability to nourish vital body organs.
Balloon angioplasty is a nonsurgical method of clearing coronary and other arteries, blocked by atherosclerotic plaque, fibrous and fatty deposits on the walls of arteries. A catheter with a balloon-like tip is threaded up from the arm or groin through an artery until it reaches the blocked area. The balloon is then inflated, flattening the plaque and increasing the diameter of the blood vessel opening. The arterial passage is thus widened. As a result of enlarging the hardened plaque, cracks may unfortunately occur within the plaque to expose the underlying fresh tissue or cells to the blood stream.
There are limitations, however, to this technique's application, depending on the extent of the disease, the blood flow through the artery, and the part of the anatomy and the particular vessels involved. Plaque build-up and/or severe re-stenosis recurrence within 6 months is up to 30-40 percent of those treated. Balloon angioplasty can only be characterized as a moderate-success procedure. Recently, a newer technique of inserting a metallic stenting element, e.g. a coronary stent, is used to permanently maintain the walls of the vessel treated at its extended opening state. Vascular stents are tiny mesh or coil tubes made of stainless steel or other metals and are used by heart surgeons to prop open the weak inner walls of diseased arteries. They are often used in conjunction with balloon angioplasty to prevent restenosis after the clogged arteries are treated. Stenting technique reduces the probability of restenosis; however, the success rate is still sub-optimal. The underlying fresh tissue or cells after angioplasty/stenting procedures still pose as a precursor for vessel reclosures, restenosis, or angio-spasm.
One major drawback with angioplasty and/or stenting is that they open up the plaque or the obstruction and expose the underlying collagen or damaged endothelium to the blood flow. Fresh collagen has pro-thrombotic and platelet-affinity properties that are part of body's natural healing processes. Unless the collagen or the damaged endothelium is passivated or modulated, the chances for blood vessel clotting as well as restenosis always exist. Moderate focal heat is known to tighten and shrink the collagen tissue. It is also clinically verified that thermal energy is capable of denaturing the tissue and modulating the collagenous molecules in such a way that treated tissue becomes more resilient. Therefore, it becomes imperative to post-treat vessels walls after the walls are treated with angioplasty, stenting, or atherectomy procedures.
One method of reducing the size of cellular tissues in situ has been used in the treatment of many diseases, or as an adjunct to surgical removal procedures. This method applies appropriate heat to the tissues, and causes them to shrink and tighten. It can be performed on a minimal invasive fashion, which is often less traumatic than surgical procedures and may be the only alternative method, wherein other procedures are unsafe or ineffective. Ablative treatment apparatus have an advantage because of the use of a therapeutic energy that is rapidly dissipated and reduced to a non-destructive level by conduction and convection, to other natural processes.
One type of thermal therapies is radiofrequency (RF) ablation, which requires tissue contact. Another type of thermal therapies is ultrasonic ablation, which can penetrate into a tissue. RF therapeutic protocol has been proven to be highly effective when used by electrophysiologists for the treatment of tachycardia; by neurosurgeons for the treatment of Parkinson's disease; and by neurosurgeons and anesthetists for other RF procedures such as Gasserian ganglionectomy for trigeminal neuralgia and percutaneous cervical cordotomy for intractable pains. Radiofrequency treatment, which exposes a patient to minimal side effects and risks, is generally performed after first locating the tissue sites for treatment. Radiofrequency energy, when coupled with a temperature control mechanism, can be supplied precisely to the apparatus-to-tissues contact site to obtain the desired temperature for treating a tissue.
Other situations may arise where it is advantageous or desirable to combine the benefits of an ultrasound ablation for a deep tissue treatment and a RF current therapy for providing focal thermal energy to the treated tissue. The RF energy can generally penetrate at about a couple of millimeters into the tissue while the ultrasound energy can generally penetrate several millimeters into the tissue. Furthermore, when RF energy is applied to an implanted stent, the stent-contacting tissue is thermally treated while the tissue gap between the stenting members is not well treated. This group of tissue gap can be simultaneously treated by ultrasound energy.
The high frequency ultrasound is the "microwave" of sound. The wavelength of the older, more conventional twenty-kilohertz ultrasound is 7.5 cm. The typical frequency is about 660 kilohertz, which has a wavelength of 2.2 millimeter. These higher frequencies allow higher ultrasonic energy densities of several hundred watts per square centimeter. Several U.S. patents disclose ultrasonic ablation and its application. Examples are U.S. Pat. No. 5,368,557 to Nita et al., U.S. Pat. No. 5,474,530 to Passafaro et al., U.S. Pat. No. 5,606,974 to Castellano et al., U.S. Pat. No. 5,676,692 to Sanghvi et al., and U.S. Pat. No. 5,827,204 to Grandia et al. However, none of the above-identified patents discloses an ablation element having dual ablation capability of ultrasound ablation and radiofrequency ablation.
Therefore, there is an urgent need for an improved ablation catheter system having the dual capability to effectively treat the tissues by using radiofrequency energy for surface treatment and using ultrasound energy for deep treatment to treat a blood vessel or other tissues, such as esophagus, larynx, liver, uterus, urethra and the like.